In the prior art, plastic parts are thermoformed continuously in groups over a width that may be quite large, e.g. as much as one meter.
Conventionally, the receptacles are filled, and then a printed flexible strip having the same width is fed continuously onto the set of receptacles and is then sealed to the flanges by an appropriate electrode or heating tool, thereby putting tops onto the receptacles. It is only after this has been done that a cutting tool is used to separate the receptacles either individually or else in groups of one or more rows (e.g. 2, 4, 6, 8, . . . ).
Naturally, while feeding a single flexible strip of tops that is preprinted, it is essential for each top to be accurately positioned relative to its receptacle. Thus, even a small amount of slip must be avoided since it will end up by accumulating and giving rise to perceptible offsets between the receptacles and the printing formed on their tops, with the printing being conventionally designed so that a well-determined print zone corresponds as accurately as possible with a receptacle.
A known solution to this problem consists in using a strip of tops at a longitudinal pitch that is slightly smaller than the longitudinal pitch of the receptacles, and in stretching the strip in the longitudinal direction and in controlled manner, to compensate for the difference in pitch. Document FR-A-1 330 729 describes a solution of that type. For tops made of materials suitable for permanent elongation, stretching is performed during a short predetermined time prior to sealing the strip onto the flanges, and this presents no particular difficulties.
However, some materials for tops, in particular composites of paper and plastic (particularly polyester) or paper and aluminum are difficult to stretch and to position since they can be lengthened to a limited extent only.
As a result, the strip must be kept under a high degree of tension throughout the process which means that forces which may give rise to failure are applied to the machine as a whole, in particular between the strip advance mechanism and the pot advance mechanism. More precisely, because of phenomena of uncontrolled slipping, wear, or jamming, it often happens that unwanted offsets arise firstly between the receptacles and the tops, and secondly between the receptacles and the heating tool for welding on the tops, or between the receptacles and the cutting tool, which naturally spoils the look of the receptacles and may even cause the strip of receptacles to rupture.
Another known solution consists in using a strip of tops having a longitudinal pitch that is slightly longer than the longitudinal pitch of the receptacles and in locally folding or wrinkling the strip of tops in order to take up the slack. Such folding or wrinkling is generally performed at the transition between two receptacles. This solution is disadvantageous, particularly with respect to appearance, especially when the receptacles are to remain in groups. Moreover, it is necessary in most cases to provide a receptacle of a special shape. Above-mentioned patent FR-A-1 330 729 teaches a solution of this type, as do U.S. Pat. Nos. 3,238,691 and 3,861,983.
The present invention seeks to mitigate these drawbacks of the prior art and to provide a method and an apparatus for placing a strip of tops on a set of receptacles in a manner that makes it possible to achieve and conserve accurate mutual positioning between the various mechanisms and tools, while avoiding exerting large forces and while avoiding the formation of folds or wrinkles in the tops, with this being done while using a strip of tops whose longitudinal pitch is slightly greater than the longitudinal pitch of the receptacles.
The invention relates in particular to improved means for re-establishing correspondence between the pitches prior to welding.